1. Field of the Invention
The invention relates generally to global navigation satellite system (GNSS) receivers and more particularly to GNSS receivers and methods for tracking weak and strong GPS signals by switching between an AFC loop and a phase lock loop.
2. Description of the Background Art
Global navigation satellite system (GNSS) receivers operate by adjusting the phase of a local replica code to the phase of a pseudorandom (PRN) code carried on an incoming GNSS signal. A global positioning system (GPS) is an example of a GNSS receiver.
The local code phase in a GNSS receiver is converted to a range between the local antenna location and the location-in-space of the GNSS satellite transmitting the GNSS signal carrying that particular code. The range is termed a “pseudorange” because the range includes local receiver clock error. In the simplest case, four pseudoranges are needed for resolving the phase offset error and computing a location fix having time and three dimensions of geographic location. Fewer than four pseudoranges can be used if some other information such as altitude or time is available.
More than four pseudoranges can be used for an over-determined solution in order to improve the accuracy of the location fix. GNSS receivers for survey and other applications that require precise locations use carrier phase and differential pseudorange corrections provided by a reference GNSS receiver.
The GNSS receiver must track the frequency of the carrier of the incoming GNSS signal before measuring the pseudorange. The carrier frequency is tracked by generating a frequency feedback adjustment to adjust the frequency of an internal local oscillator (LO) signal (or combination of LO signals) to match the frequency of the carrier. It is important that the LO signal frequency have as little phase noise as possible because phase noise on the LO signal causes noise on the pseudoranges. Noise on the pseudoranges results in less accurate location fixes.
Existing GNSS receivers use a phase lock loop (PLL) for generating a low noise LO signal. However, phase lock loops can cycle-slip or lose lock unless the GNSS signal is relatively strong. In order to operate with weak GNSS signals, some GNSS receivers use an automatic frequency control (AFC) loop for the LO signal. The LO signal frequency provided with an AFC loop is noisier than with a phase lock loop, and the AFC loop does not output the higher accuracy carrier phase measurements, but the AFC loop operates more reliably at lower GNSS signal strengths.
Phase locking with a phase lock loop may take several seconds in order for the phase lock loop to settle to an equilibrium state. During this time period the pseudorange measurement is not available. When that pseudorange measurement is one of the minimum set that is being used for a location fix (four in the simplest case) then no location fix is available during the time that the phase lock loop is settling.
Even when the location fix is using more than the minimum set of pseudoranges, the location fix is subject to jumps before and after the phase lock loop settles when the GNSS receiver makes the transition to the phase lock loop. These problems may be tolerable when the signal strength does not often change between weak and strong. Unfortunately, there are cases where the GNSS signal strength changes rapidly such as when the GNSS signal is subject to multipath or the GNSS receiver is beneath a tree canopy. When the GNSS receiver is beneath a tree canopy, the loop transition problem may occur each time the GNSS satellite or the GNSS receiver moves just enough to pass by a branch or leaf in the line-of-sight between the satellite and the receiver.